Submarine faults strongly impact age and salinity distributions in offshore freshened groundwater systems

Abstract

Coastal aquifers provide a vital connection between land and ocean, where the dynamics of groundwater flow and solute transport are influenced by confounding effects from hydrogeologic, morphologic, climatic, and oceanic forcings. Most previous studies of coastal and subsea aquifers have focused on aquifers with continuous structures, with few addressing submarine faults that can disrupt this continuity and play a controlling role in groundwater flow, salinity and age distributions. We investigate the understudied influence of submarine faults by simulating a generalized coastal aquifer-aquitard system with a fault cutting through the system, with simulations accounting for realistic sea-level rise over paleohydrogeologic timescales. Results show that faults reduce the extent of offshore freshened groundwater (OFG) regardless of their location. Faults proximal to the coastline serve as pathways for freshwater discharge, accounting for up to 18.1% of total submarine groundwater discharge (SGD), and transport groundwater that is 7 times older than the surrounding SGD. Conversely, faults located farther offshore act as conduits for downward seawater infiltration, rejuvenating deep aquifers by a negative peak of 2000 years with saline water. The two contrasting processes may regulate the chemical loads of groundwater through the seafloor. The dip of the aquitard has little impact on OFG extent but enhances the flux of groundwater discharge through the fault to the sea. The control of faults over groundwater flow peaks when the fault is oriented vertically. The findings suggest that special attention should be paid to submarine faults, which can strongly influence fresh groundwater resources and biogeochemical reactions in coastal/marine areas.